The Schmitt trigger is a variation of the simple comparator which has
hysteresis, that is, it has a toggle action. When the output is high,
positive feedback makes the switching level higher than it is when the
output is low. A little positive feedback makes a comparator with better
noise immunity. Increase the positive feedback and the Schmitt trigger can
be used in other switching applications. Look at the Schmitt trigger
circuit shown at right. Notice that va = [R1/(R1 + Rf)] vo, it depends on
the output. Letâ€™s say the output is low and the input is decreasing. When
vin < va the output goes high and suddenly va goes a little bit higher
with it. That makes the difference between vb and va even bigger. To make
the circuit switch again vin has to go back up beyond the original
switching level. It has to reach the new va before the output will switch
low. In this circuit the two switching levels are above and below ground by
the same amount (unless you have non symmetric power supplies). The
circuits above are variations of the Schmitt trigger. In the first circuit,
the input is again compared to levels above and below ground, but this time
the output goes high when the input goes high and vise-versa. In the
remaining circuits the switching levels are not symmetric about 0 V, but
about some voltage set by the voltage divider of R1 and R2.

CIRCUIT DIAGRAM:

WAVE FORMS:

PROCEDURE:

1. Connect the circuit as per the circuit diagram.

2. For a zero crossing detector, connect the non-inverting terminal to
ground.

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